Stenotic lesions in human arteries are dilated using balloon catheters. A balloon catheter typically consists of a long, elongated tube having a strong elongated balloon member attached to its distal end. The balloon in most cases can be inflated independently so that the lesion is compressed so as to open the stenotic lesion. This procedure is called angioplasty. Balloon catheters are made in balloons having different diameters, for example, 2.0 mm to 10.0 mm in diameter. Angioplasty is performed in both coronary arteries, i.e., arteries supplying blood to the heart muscle, as well as in other arteries of the body. When angioplasty is performed in coronary arteries, it is called PTCA--percutaneous transluminal coronary angioplasty--and when performed on other vessels, it is called PTA--percutaneous transluminal angioplasty.
In either case, after balloon angioplasty, the vessel may reclose due to many factors. The early or acute reclosure or late or restenosis is one of the most important limitations of balloon angioplasty. At present, devices called "stents" are placed after or during balloon angioplasty to maintain the patency of the vessel. Stents are metallic or nonmetallic structures that are placed in the artery at the lesion location to maintain the patency. Several stents have been developed and are described in the literature. U.S. Pat. No. 4,649,922 by Wiktor; U.S. Pat. No. 4,733,665 and U.S. Pat. No. 4,739,762 by Palmaz, et al.; U.S. Pat. No. 4,800,882 by Gianturco, et al.; U.S. Pat. No. 4,954,126 by Wallstein, et at., describe a few of the stents that have been developed for the purpose of stenting the vascular lesion to maintain patency post-balloon angioplasty or mechanical atheractomy.
The outcome of vascular stenting is under several clinical studies, both in the U.S. and in Europe. One of the main complications is the delivery of the stent to the lesion location. In the United States, stents such as the Palmaz-Schatz stent by Johnson & Johnson International Systems (JJIS) is sold mounted on a delivery catheter. Similarly, the Gianturco-Roubin stent made by Cook Inc. is also sold mounted on a delivery catheter. In either case, the delivery catheter is a balloon catheter, similar to a normal PTCA or PTA balloon catheter. The metallic stent, the Palmaz-Schatz or the Gianturco-Roubin stent, is mounted on the folded balloon by sliding the stent onto it. The stent is held in place by a polymer sheath that extends the entire length of the catheter body. In Europe, the stent is sold separately. When the physician decides to place a stent in the dilated stenotic lesion, he mounts the stent on the folded balloon so as to deploy it at the lesion.
It is known in the literature that often the unprotected stent has a tendency to slide off the balloon portion of the catheter which can result in losing the stent inside the arteries of the body. It is, therefore, quite important that the stent is properly secured to the stent delivery system, be it a balloon catheter or another system for safe delivery and deployment of the stent. For this reason, the stent sold by JJIS is mounted on a balloon catheter and the stent is secured in place by a sheath. The sheath is placed substantially over the entire length of the stent and proximally the sheath extends toward the proximal end of the balloon catheter. Often, the sheath proximally is attached to a "Y" adapter. The entire sheath assembly can be slid over the balloon catheter and moved back and forth. By retracting the sheath, the stent can be exposed for deployment.
In a typical procedure, the physician performs balloon angioplasty on a stenotic lesion. The primary catheter used for dilation of the lesion is then with-drawn using an extension guidewire. The catheter (balloon-mounted stent delivery catheter) is then threaded to the lesion location so that the stent is at the location as determined by fluoroscopic X-ray. The sheath is then slid back sufficiently to expose the stent. The balloon is then inflated to deploy the stent and the balloon is subsequently deflated and the entire apparatus is withdrawn, leaving a guidewire in place. A third catheter having a high-pressure balloon is then threaded beneath the stent and is inflated to higher pressure (16-22 atmospheres) to seat and embed the stent into the intimal wall of the artery.
Several devices have been developed and invented to deliver stents. One such developed by JJIS was earlier discussed. This system has the disadvantage that the delivery catheter and the accompanying sheath have a very large diameter and are relatively stiff in the area near the stent. When the distal 20-30 cm of the catheter is stiff, it makes it very difficult and often impossible for the catheter to track the guidewire to tortuous vessels in the body. This often limits the use of stents to proximal lesions of the coronary arteries. Secondly, due to the fact that the sheath makes the profile of the catheter quite large (0.050-0.070" in some cases), it is often difficult to cross even those lesions that have been previously dilated.
Other devices have been invented for the delivery of stents. Such a device is described in U.S. Pat. No. 5,108,416 by Ryan and Chiev. This concept appears to resolve the problem of a stent sliding off the balloon by the use of holding cups, resulting in a stent delivery catheter having two (or one) bulky structures on the balloon at the location of the stent.
In order for a catheter to track well into the tortuous vasculature, it is quite important that the apparatus is quite flexible and it has neolithic bending characteristics. This means that the catheter body should not have areas that are bulky so that when the catheter is bent into different planes three dimensionally, it bends evenly without sharp bends. The structure described in U.S. Pat. No. 5,108,416 by Ryan, et al, when threaded over a guidewire bends unevenly, causing friction between the inner diameter of the catheter and the guidewire, making it impossible for this apparatus to be threaded to more tortuous arteries.
U.S. Pat. No. 5,158,548 by Lau and Hartigan describes an apparatus where a sheath-type device, similar to that described in U.S. Pat. No. 5,108,416, where the sheath or "holding cup" is attached to a central guidewire and the guidewire is advanced forward to expose the stent. This design has several drawbacks. First, it is necessary that the stent delivery catheter is advanced over a guidewire into position. The fact that the central lumen of the apparatus described is occupied by the wire described in the invention does not permit the placement of the stent delivery system over a guidewire. For safety reasons, most catheters, including balloon catheters and stent delivery catheters, are advanced over a guidewire. Catheters such as the one described in U.S. Pat. No. 5,158,548 by Lau, where a small guidewire portion is attached to a bulky sheath, often can cause perforations of the vasculature or can break plaque loose, causing severe complications, including death, when such happens in the coronary artery. Secondly, this design causes the distal portion of the catheter to be hard and bulky, and does not promote easy tracking of the catheter/balloon/stent into tortuous vasculature.
In addition, other methods and variations of these are present in the literature. In all of these cases, the inventors have failed to address the practical considerations due to the smallness and the complex and tortuous vasculature of the human arteries, especially the coronary arteries.